Many applications exist for compact, low cost, solid state lasers which can lase at wavelengths in the green, blue and ultraviolet (UV) regions of the spectrum. Some of the major applications include optical data storage and retrieval, color printing, color displays, and medical analysis. Various techniques are currently being pursued in order to produce compact, low cost, solid state green, blue, and ultraviolet lasers. The three primary techniques involve frequency doubling of infrared semiconductor laser diodes, the development of semiconductor laser diodes based in the zinc selenide system, and lasers based on upconverting photon addition phosphor (upconversion) materials. Although green and blue lasers have been demonstrated with frequency doubling techniques, there are still questions remaining as to their robustness, compactness, practicality and cost. Although several groups have demonstrated semiconductor laser diodes based in the zinc selenide system which lase in the blue-green region of the spectrum these semiconductor laser diodes are currently operating at performance levels well below that necessary for commercial deployment.
Recently, lasers based on upconversion materials have been demonstrated that are capable of lasing at wavelengths in the red (R), green (G), and blue (B), collectively referred to as RGB, regions of the spectrum. The upconversion materials used in these lasers are based on rare earth doped fluorides which have been produced as bulk single crystals or fluoride glass fibers. Infrared wavelength radiation absorbed by these materials is internally transferred from the absorbing species to other species in a number of steps where excited states result in emitted visible light and, in some cases, ultraviolet radiation when they decay to other lower lying energy states. Because of the wavelengths involved in the initial absorption, lasers based on these materials can be pumped by conventional semiconductor laser diodes which emit radiation at infrared or red wavelengths.
The art recognizes the metal fluoride material system Ba-Ln-F, where Ln is Yttrium (Y), Ytterbium (Yb), Praseodymium (Pr), Holmium (Ho), Erbium (Er), Thulium (Tm), or a combination thereof, as the preferred materials to convert longer wavelength radiation into shorter wavelength radiation having one or more visible or ultraviolet wavelengths by the upconversion process of photon absorption followed by emission. (See F. Auzel and D. Pecile, J. of Luminescence 8, 32 (1973)). In particular, the art recognizes the rare earth ion Yb as a sensitizing agent which can absorb infrared wavelength radiation (in the approximate range of 950 nm to 980 nm) and transfer part of that energy to one or more of the upconversion dopant species (i.e., Pr, Ho, Er, Tm). Depending on the pump wavelengths (more than one pump wavelength may be required) as well as the dopants (more than one dopant may be used) visible radiation at wavelengths in the red, green, blue, and ultraviolet regions of the spectrum have been obtained.
A specific example is the metal fluoride material of nominal composition BaYYb.sub.0.99 F.sub.8 doped with 1% Tm (by atomic weight), which will absorb radiation at 960 nm, and transfer part of this energy to the Tm ions through a series of multi-photon excited state absorptive steps. The excited ions will then decay to lower energy states and emit radiation in the approximate ranges of 350 nm to 370 nm, 440 nm to 490 nm, 500 to 520 nm, 630 nm to 670 nm, and 760 nm to 840 nm. The art has also shown it possible to directly pump the Tm ions by using radiation at a wavelength in the range of 645 nm to 665 nm or 675 nm to 685 nm. (See G. Ozen, J. P. Denis, Ph. Goldner, Xu Wu, M. Genotelle, and F. Pelle, Appl. Phys. Lett. 62, 928 (1993)). Other metal fluoride host/rare earth dopant combinations may be used to effectively convert a portion of longer wavelength radiation to shorter visible or ultraviolet wavelength radiation. For example, the addition of Er to the above system would result in the emission of radiation in the approximate ranges of 540 nm to 560 nm and 630 nm to 670 nm.
An upconversion laser system for the conversion of infrared wavelength radiation to relatively shorter wavelengths comprising a host doped with a rare earth activator, a resonant optical cavity, and a pump source has been detailed in the European Patent Application No. 534,750 to Thrash. An upconversion process for the conversion of infrared wavelength radiation in the range of approximately 1080 to 1300 nm to relatively shorter wavelengths which involves a glass host, including optical fibers, having a concentration of rare earth activator ions is detailed in U.S. Pat. No. 5,226,049 to Grubb.
Although lasers made with bulk crystalline or glass hosts and fiber upconversion material fulfill the spectral requirements for many of the applications listed previously, they may be limited in their scope due to potential high costs associated with materials, packaging, non-compactness of the configuration, and the lack of monolithic integration with other devices.